The use of a transformer controlled by a switching mode circuit ("switching mode transformer") in a power supply circuit as a "DC/DC" converter is well known in the field of electronic electricity meters. The main capacitor is connected and disconnected alternately from the primary of the transformer by the switching circuit to generate a voltage at the secondary of the transformer. The voltage obtained in this way is filtered and supplied to the measuring circuit. The switching mode transformer is controlled in such a way as to supply a relatively constant voltage to the measuring circuit.
The voltage is typically controlled by means of a return coil that monitors the secondary voltage and sends a control signal to the switching circuit to control the time for which the main capacitor is connected to the transformer. The return winding is typically wound around the core between the primary and the secondary. The return winding is closely coupled to the secondary to monitor precisely the variation in the voltage at the outputs of the secondaries in order to generate the control signal sent to the switching circuit.
Note that the earth of the measuring circuit is often different from that of the power supply circuit. For example, for a multiphase meter adapted to operate in the event of loss of the neutral, the main capacitor will be supplied with power by a current resulting from rectification of all the phases and the neutral relative to an earth which is common to the rectifier and to the power supply circuit and which is different from neutral. On the other hand, the earth of the measuring circuit is the neutral.
In the event of an interruption of the current, the power supply to the measuring circuit is assured for a period of time by the charge stored in the capacitor. To identify an interruption of the current and to determine the remaining quantity of charge, the measuring circuit must monitor how the voltage across the main capacitor varies. Given that the earth of the power supply circuit and that of the measuring circuit are not the same, it is not possible for the measuring circuit to measure the voltage directly.
In conventional systems, the voltage is measured via an isolating interface between the two circuits, for example by using an optocoupler or a differential amplifier adapted to measure the potential difference across the capacitor. These solutions are relatively costly and necessitate the use of a large number of components to define the interface.